Cant say I recall anything in the 70s that shout "Peter Parker is in his late 20s". He pretty much stopped aging in real time by that point, no?

I can't cite point for point evidence, but my "feel" of it has always been a pretty natural progression in the mainstream Marvel universe.

1960's - Pete in high school / early college
1970's - Pete's college age years
1980's - Pete in mid 20's
1990's - Pete in mid to late 20's
2000's - Pete around age 30

Of course, the recent "Brand New Day" kind of throws a curve ball in things; I can't really get a handle on where things are. I suppose he's supposed to be exactly where he was when Stracynski's book left off; but for me, "Brand New Day" feels like a regression in age back to mid 20's.